Tektronix MSO2000 Series Manual de usuario

Introduction to Oscilloscopes

Lab Experiment

A collection of lab exercises to introduce you to the basic

controls of a digital oscilloscope in order to make common

electronic measurements.

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Introduction to Oscilloscopes Lab Experiment

Table of Contents

LABORATORY EXPERIMENT INTRODUCTION........................................................................................ 3

OBJECTIVES ............................................................................................................................................... 3

EQUIPMENT LIST......................................................................................................................................... 3

OVERVIEW OF AN OSCILLOSCOPE......................................................................................................... 4

INTRODUCTION............................................................................................................................................ 4

PERFORMANCE TERMS AND CONSIDERATIONS ............................................................................................. 5

INITIAL SETUP AND SCREEN EXPLANATION......................................................................................... 6

CREATING A STABLE DISPLAY..................................................................................................................... 6

SCREEN EXPLANATION ............................................................................................................................... 7

INSTRUMENT CONTROLS ......................................................................................................................... 8

VERTICAL CONTROLS...............................................................................................................................9

INTRODUCTION............................................................................................................................................ 9

VERTICAL POSITION/SCALE CONTROLS ....................................................................................................... 9

HORIZONTAL CONTROLS ....................................................................................................................... 10

INTRODUCTION.......................................................................................................................................... 10

HORIZONTAL POSITION/SCALE CONTROLS................................................................................................. 10

SETTING THE RECORD LENGTH ................................................................................................................. 10

TRIGGER CONTROLS...............................................................................................................................11

INTRODUCTION.......................................................................................................................................... 11

TRIGGER LEVEL CONTROL ........................................................................................................................ 11

TRIGGER MENU ........................................................................................................................................ 12

OSCILLOSCOPE MEASUREMENTS........................................................................................................ 13

INTRODUCTION.......................................................................................................................................... 13

MANUAL MEASUREMENTS ......................................................................................................................... 14

CURSOR MEASUREMENTS ......................................................................................................................... 14

AUTOMATED MEASUREMENTS ................................................................................................................... 15

FINAL EXERCISE ...................................................................................................................................... 16

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Introduction to Oscilloscopes Lab Experiment

Laboratory Experiment Introduction

Objectives

1. Understand the block diagram and basic controls of a digital oscilloscope.

2. Setup an oscilloscope for a stable display of the applied signal.

3. Make common electronic measurements with a digital oscilloscope.

Equipment List

1. One Tektronix MSO2000 or DPO2000 Series digital oscilloscope.

2. One Tektronix P2221 1X/10X passive probe.

3. One Host/Device USB cable.

4. One Tektronix 878-0456-xx demonstration board.

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Introduction to Oscilloscopes Lab Experiment

Overview of an Oscilloscope

Introduction

An oscilloscope is an electronic test instrument that displays electrical signals graphically, usually as a

voltage (vertical or Y axis) versus time (horizontal or X axis) as shown in figure 1. The intensity or

brightness of a waveform is sometimes considered the Z axis. There are some applications where other

vertical axes such as current may be used, and other horizontal axes such as frequency or another

voltage may be used.

Oscilloscopes are also used to measure electrical signals in response to physical stimuli, such as

sound, mechanical stress, pressure, light, or heat. For example, a television technician can use an

oscilloscope to measure signals from a television circuit board while a medical researcher can use an

oscilloscope to measure brain waves.

Oscilloscopes are commonly used for

measurement applications such as:

•observing the wave shape of a signal

•measuring the amplitude of a signal

•measuring the frequency of a signal

•measuring the time between two events

•observing whether the signal is direct

current (DC) or alternating current (AC)

•observing noise on a signal

An oscilloscope contains various controls that assist in the analysis of waveforms displayed on a

graphical grid called a graticule. The graticule, as shown in figure 1, is divided into divisions along both

the horizontal and vertical axes. These divisions make it easier to determine key parameters about the

waveform. In the case of the MSO/DPO2000 Series oscilloscope, there are 10 divisions horizontally

and 8 divisions vertically.

A digital oscilloscope acquires a waveform by conditioning the input signal in the analog vertical

amplifier, sampling the analog input signal, converting the samples to a digital representation with an

analog-to-digital converter (ADC or A/D), storing the sampled digital data in its memory, and then

reconstructing the waveform for viewing on the display.

Figure 2: Typical Digital Oscilloscope Block Diagram

Figure 1: Typical Oscilloscope Display

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Introduction to Oscilloscopes Lab Experiment

Performance Terms and Considerations

There are many ways to specify digital oscilloscope performance, but the most important are bandwidth,

rise time, sample rate, and record length.

Bandwidth

Bandwidth is the first specification to consider. Bandwidth is the frequency range of the oscilloscope,

usually measured in Megahertz (MHz). It is the frequency at which the amplitude of the displayed sine

wave is attenuated to 70.7% of the original signal amplitude.

When measuring high-frequency or fast rise-time signals, oscilloscope bandwidth is especially critical.

Without adequate bandwidth, an oscilloscope will not be able to display and measure high-frequency

changes. It is generally recommended that the oscilloscope’s bandwidth be at least 5 times the highest

frequency that needs to be measured. This “5-times rule” allows for the display of the 5th harmonic of the

signal and assures that measurement errors due to bandwidth are minimized.

signalofharmonicbandwidthpeoscillosco th

5≥

Example: If the signal of interest is 100 MHz, the oscilloscope would need a bandwidth of 500 MHz.

Rise Time

The edge speed (rise time) of a digital signal can carry more high-frequency content than its repetition

rate might imply. An oscilloscope and probe must have a sufficiently fast rise time to capture the higher

frequency components, and therefore show signal transitions accurately. Rise time is the time taken by

a step or a pulse to rise from 10% to 90% of its amplitude level. There is another “5-times rule” that

recommends that the oscilloscope’s rise time be at least 5 times faster than the rise time of the signal

that needs to be measured.

5timerisesignal

timerisepeoscillosco ≤

Example: If the signal of interest has a rise time of 5

sec, then the oscilloscope rise time should be

faster than 1

sec.

Sample Rate

Digital oscilloscopes sample the input signals at a frequency called the sample rate, measured in

samples / second (S/sec). To properly reconstruct the signals, Nyquist sampling requires that the

sample rate be at least twice the highest frequency being measured. That’s the theoretical minimum. In

practice, sampling at least 5 times as fast is generally desirable.

Highest

fratesample ∗≥ 5

Example: The correct sample rate for a 450 MHz signal would be ≥2.25 GS/sec.

Record Length

Digital oscilloscopes capture a specific number of samples or data points, known as the record length,

for each acquired waveform. The record length, measured in points or samples, divided by the sample

rate (in Samples/second) specifies the total time (in seconds) that is acquired.

ratesample

lengthrecord

timeacquired =

Example: With a record length of 1 Mpoints and a sample rate of 250 MS/sec, the oscilloscope will

capture a signal 4 msec in length.

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Introduction to Oscilloscopes Lab Experiment

Exercise

What minimum oscilloscope performance is required to properly capture 2 msec of a 1 Vpk-pk , 250 MHz

sine wave?

•Bandwidth:

•Sample Rate:

•Record Length:

Initial Setup and Screen Explanation

Creating a Stable Display

1. The following steps will describe how to automatically create a stable oscilloscope display using a

1 kHz, 5 Vpk-pk square wave.

a. Power up the MSO/DPO2000 Series oscilloscope by pressing the power button on the lower

left corner of the instrument.

b. Press the front panel Default Setup button to set the oscilloscope to a known starting point.

c. Connect a P2221 1X/10X passive probe to the channel 1 input. To connect a BNC connector,

push and turn the probe connector until it slides on the connector. Then, turn the locking ring

clockwise to lock the connector in place.

d. Use the probe slide switch to set the probe attenuation to 10X.

e. Attach the probe’s alligator style ground lead to the ground connector on the lower right

corner of the oscilloscope.

f. Attach the probe tip to the PROBE COMP connector just below the ground lead connector. The

PROBE COMP connector provides a 1 kHz square wave that this lab will use to demonstrate

the operation of an oscilloscope.

g. Press the front panel Autoset button to

cause the oscilloscope to automatically

set the vertical, horizontal and trigger

settings for a stable display of the

PROBE COMP 1 kHz square wave.

Key Points to Remember

1. To return the oscilloscope to a known state, press the Default Setup button.

2. The Autoset button adjusts the vertical, horizontal and trigger settings such that four or five cycles

of the waveform are displayed with the trigger near the middle of the screen.

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Introduction to Oscilloscopes Lab Experiment

Screen Explanation

1. Following is a review of the oscilloscope’s display.

a. The channel 1 vertical axis button is yellow and most of the elements on the screen that relate

to the channel 1 signal are yellow in color.

b. On the display, the following items are yellow to indicate they are associated with channel 1:

•waveform

•waveform ground level indicator (center left of screen)

•vertical scale readout (bottom left of screen 2.00 V)

c. The channel 2, 3, and 4 vertical axis buttons are blue, magenta and green respectively. The

display uses the color coding of these channels just as it does for the yellow of channel 1.

d. As can be seen on the oscilloscope screen, the square wave extends up about 2 ½ divisions on

the display graticule from the ground level indicator. Since the vertical scale factor is 2

Volts/div, this indicates the signal’s positive peak is at about +5 V.

e. One cycle of the waveform is about 2 ½ divisions wide. The time per horizontal division is

indicated by the horizontal scale readout which in this case is 400 µsec/div (bottom center of

the display). At 400 µsec/div, the period of the signal is about 1 msec and the frequency is

about 1 kHz.

f. Finally, the trigger frequency readout indicates the channel 1 signal has a frequency of about

1 kHz as shown in the bottom right corner of the display.

Key Points to Remember

1. The input channels are color coded. Onscreen channel information is in that channel’s color,

including the waveform, ground indicator, and vertical scale factor (Volts/div).

2. The amplitude of the signal can be determined by multiplying the number of vertical divisions the

waveform spans times the vertical scale factor.

3. The signal period can be determined by multiplying the number of horizontal divisions times the

horizontal scale factor.

4. Signal frequency is calculated by dividing 1 by the signal period.

Exercise

Based on the display shown here, answer the

following questions:

What is the peak-to-peak voltage of the signal?

What is the voltage of the signal’s positive

peak? Negative peak?

What is the period and frequency of the signal?

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Introduction to Oscilloscopes Lab Experiment

Instrument Controls

The controls of a typical oscilloscope can be grouped into three major categories: vertical, horizontal,

and trigger. These are the three main functions that are used to set up an oscilloscope. The use of

these controls is described in the following sections of this lab.

Vertical Controls

Trigger

Controls

Horizontal Controls

Here are a few hints that will make using the oscilloscope controls easier:

•Decide if the task is related to oscilloscope’s vertical axis (typically voltage), horizontal axis

(typically time), trigger, or some other function. This will make it easier find the correct control or

menu.

•Pressing a front panel button will usually display a first-level menu at the bottom of the display.

The menu items are logically prioritized from left-to-right. If they are selected in that order, the

setup should be straightforward.

•In most cases, pressing the button underneath a menu item at the bottom of the display results

in a second-level menu at the side of the display. These menu items are logically prioritized top-

to-bottom.

•If a small orange aor bis displayed on the screen, it indicates the front panel Multipurpose a

or bcontrols may be used to change that menu selection.

•Pressing the Menu Off button turns off one menu level at a time until all menus and readouts

are removed.

Exercise

The oscilloscope’s vertical axis controls are typically used to control which parameter?

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Introduction to Oscilloscopes Lab Experiment

Vertical Controls

Introduction

The vertical controls set or modify the vertical scale, position, and other signal conditioning for each of

the analog input channels.

There is a set of vertical controls for each input channel. These controls are used to scale, position, and

modify that channel’s input signal so it can be viewed appropriately on the oscilloscope display. In

addition to the dedicated vertical controls for each channel, there are also buttons to access the math

menu, reference menu and bus menus.

Vertical Position/Scale Controls

1. The following steps will explore the use of the vertical axis position and scale front panel controls.

a. Use the channel 1 vertical Position knob to position the waveform near the bottom of the

display and notice the ground level indicator also moves.

The vertical position control moves the waveform up and down. It is generally used to align the

waveform with the vertical divisions on the graticule. Position is generally a graphical display

function only and does not affect the acquired waveform data.

b. Use the channel 1 vertical Scale knob to change the vertical scale from 2 V/div to 1 V/div.

The vertical scale (Volts/division) control adjusts the height of the waveform on the display.

Generally, the vertical scale control changes the settings of the input amplifier and/or attenuator

and does affect the acquired waveform data. Because the vertical scale controls the amplitude

of the signal going into the ADC, the highest-resolution measurements are achieved when the

signal almost fills the screen vertically without going off screen.

Key Points to Remember

1. The vertical position knob controls the position of the waveform on the vertical axis.

2. The vertical scale knob controls the amount of voltage represented by a vertical division on the

graticule.

Exercise

To make the highest-resolution measurement, what vertical scale should be used to measure the

PROBE COMP square wave? Why?

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Introduction to Oscilloscopes Lab Experiment

Horizontal Controls

Introduction

The horizontal controls are used to scale and position the time axis of the oscilloscope display. There is

a dedicated front panel control for setting the horizontal scale (time/division) of the display and another

for setting the horizontal position of the displayed signals. The Acquire menu offers additional options

for modifying the waveform display, as well as setting the record length.

Horizontal Position/Scale Controls

1. The following steps will explore the use of the horizontal axis scale front panel control. The

horizontal scale control (also known as time/division or seconds/division) adjusts the amount of time

displayed on the screen.

a. Press the front panel Autoset button to restore the oscilloscope to a known starting point and

then set the vertical scale to 1 V/div.

b. Use the vertical Position knob to center the waveform on the screen.

c. Turn the horizontal Scale knob until the horizontal readout indicates 10μs/div (readout is shown

in the bottom center of the display.)

Since there are 10 divisions horizontally, a scale factor of 10 µsec/div yields a 100 µsec time

window. This setting shows the actual shape of the rising edge of the square wave.

2. The horizontal Position control moves the waveform and its horizontal reference or trigger point

(indicated by the orange icon at the top of the display) back and forth on the display. This is used to

align the displayed waveform with the horizontal divisions on the display graticule.

a. Turn the horizontal Position knob counter-clockwise to position the waveform’s falling edge at

the center of the display.

Key Points to Remember

1. The horizontal scale control sets the time window displayed on the oscilloscope screen. Since there

are 10 divisions horizontally, the time window is equal to: divisionsfactorscalehorizontalwindowtime 10∗=

2. The horizontal position knob allows you to align the displayed waveform with the horizontal divisions

of the display graticule or to view a different section of the displayed waveform.

Exercise

If the horizontal scale factor were set to 1 μsec/div, the displayed time window would be:

Setting the Record Length

1. The following steps will investigate the relationship between the oscilloscope’s horizontal scale

factor, record length and sample rate.

a. Set the horizontal scale to 100 μs/div.

b. Press the Acquire front panel button. Press the Acquisition Details bottom bezel button.

Notice that the sample rate is currently 125 MS/s for a record length of 125 kpoints.

c. Press the Record Length bottom bezel button and press the 1.00M points side bezel button.

This sets the record length to 1 Mpoints.

d. Press the Acquisition Details bottom bezel button again. Notice that the sample rate is now 1

GS/s. The 1 msec time window has not changed, meaning the sample rate was increased by

the same ratio as the record length.

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